2000 GMC C1500 Service Engine Soon Light

My Service Engine Soon light came on. I took my truck to the GMC Dealer. He said it needed a new O2 sensor along with a new fuel filter. He also suggested new spark plugs and plug wires. I had them change the fuel filter and O2 sensor. I installed the new spark plugs and plug wires. The light came back on 2 days later. I took it back to the dealer and they said it needed a new catalytic converter. Took it to the Midas shop and they said it was reading low voltage to the O2 sensor and the converter had the same temp reading before and after the converter. The converter had only 17,000 miles on it. I took it back to the dealer and they replaced the O2 sensor again but said they thought there was still a problem with the converter. The dealer reset the service engine soon light. It came back on on the way home from the dealer. Took it back to the Midas Muffler shop and they replaced the converter even though the reading was a low voltage reading on the O2 sensor. Was half way home and the service engine soon light came back on again. I have now taken it back to the dealership and they say they still think the catalytic converter is the problem. The new converter in bank 2 is the exact same size as the converter on bank 1. The converter on bank 1 is the original. This is the 4th one on bank 2. The reading is still a low voltage issue with the O2 sensor. Could someone please help?

Thanks!